This invention relates to an apparatus and a process for deforming materials, and, more particularly, to an apparatus and a process for continuously and progressively forming composite material workpieces.
A composite material combines two or more other materials into a single integrated material structure, in a manner whereby the combined materials retain their original identities. One of the best known, commercially most important types of composites is the fiber composite material formed of long, substantially continuous high-strength fibers incorporated into a metallic or a polymeric matrix. These composites typically have large numbers of fibers embedded in the matrix. The fibers are usually selected to be strong, but are of low elongation to failure. Examples of the materials used as high-strength fibers include carbon, graphite, glass, "KEVLAR", aramid boron and silicon carbide.
The matrix holds the fibers in the proper orientation and protects them from external damage. The matrix may be a metal or nonmetal, such as a polymer. Polymeric mattix materials fall into two general classes, thermosetting and thermoplastic. Most thermosetting polymers are not readily formable after curing by deformation processes. However, thermoplastic polymers can be plastically deformed, at sufficiently high temperatures, to a permanent change in shape without fracture because they do not require curing.
The present invention relates to fiber composite materials having long fibers embedded in a plastically deformable matrix. Examples of such types of composites include metal and polymer matrix composites having long fibers. At the present time one of the most important types of such composites is prepared with carbon or glass fibers embedded in a matrix having a high temperature operating capability, such as polyetheretherketone (PEEK). The development of such fiber composite materials is continuing, with new combinations having improved properties emerging each year.
The most commonly used approach to making articles of the composite material is to lay up tapes of fiber-containing matrix material into dies, and then to consolidate the tapes by pressing the tapes with another die in a large heated press. Consolidation can also be accomplished with a single die in an autoclave. To achieve good strength and stiffness properties in the finished material, the fibers in the layers may be oriented in various directions within the plane of the workpiece. The consolidated laminate therefore consists of discrete layers or plies. Within each ply there is an intimate mixture of fiber and matrix material, but between layers there is often some additional matrix material.
The press used to consolidate the tapes can utilize flat platen dies to press the layers of composite material, to produce a flat plate of the composite material. Flat plates have limited uses, because most components are not flat, and instead contain curved sections. Shaped pieces of composite can be made by substituting curved fixed dies for the flat platens, so that the composite is consolidated to the desired final shape. Alternatively, pieces of the composite can be consolidated as flat plates, and then reshaped using fixed dies corresponding to the overall final shape of the component, in a separate die forming operation. This approach is termed post forming.
The fabrication of composite structures using fixed dies has significant drawbacks and limitations, both from technical and economic standpoints. Forming in a press having dies for platens often results in gaps and voids in the structure, particularly between layers, because of the difficulties in achieving the proper uniform layup of the tapes prior to pressing and a uniform pressing pressure during die forming. The post-forming approach may produce high compressive stresses in the fibers in the concave-going side of bends, resulting in buckling and misalignment of the fibers and unsatisfactory mechanical properties. Economic drawbacks to the use of the die-forming approach are equally significant.
In a recently developed alternative approach, a fiber composite workpiece is progressively formed into a shaped piece in a die-less forming operation. The workpiece is formed by passing it through a forming apparatus in which a bend or curve is introduced into the workpiece, and that bend is propagated throughout the workpiece. This approach has the important advantage of achieving the formed shape by a kinematically admissible technique wherein the lengths of the composite laminae remain constant during the forming operation, thereby avoiding the laminar buckling and fiber breakage problems that occur during press forming. The die-less forming technique is disclosed in U.S. Pat. Nos. 4,777,005 and 4,955,803, whose disclosures are incorporated by reference.
Although the die-less forming technique provides a major advance over the die-forming techniques and is successfully employed in many situations, it may have technical and economic limitations in others. There may be limitations on the fabrication of particular shapes and bends. In some cases, there are limits on the speed of the forming operation, affecting the process economics.
Thus, there remains a need for an improved forming operation that produces high-quality, defect-free articles and parts, in an economically efficient manner. The present invention fulfills this need, and further provides related advantages.